Denervation and muscle inactivation by long-term anesthesia of the motor nerves have been reported to have almost identical effects in rats but different effects in rabbits. The proposed work is intended to reinvestigate the problem and to find out whether the muscle membrane prperties are controlled by one factor giving rise to a multiplicity of phenomena or by several factors which may act independently and in varying degrees in various animals. Another problem related to the first is the question of the origin of the ACh receptors in denervated and inactivated muscles. Are the ionic channels in the muscle membrane which are associated with the extra junctional ACh receptors identical with preexisting cation-transporting channels? The existing evidence, which shows that it is mainly the anionic conductance which decreases following denervation in mammalian muscles is against such as idea. The proposed investigation is intended to explore in more detail the changes in cationic and anionic membrane conductances which take place following denervation and inactivation in mammalian skeletal muscles.